Inorganic materials have long been used as fillers, pigments, reinforcements and chemical reactants in polymers. In general, these inorganic materials are hydrophilic, that is, easily wetted by water or able to absorb water, but their compatibility with organic polymers is limited. Because of this limited compatibility, the full potential of color, reinforcement, or chemical reactivity of the inorganic materials is not realized.
To overcome these difficulties, wetting agents have been used to minimize interfacial tension: but wetting agents, too, have serious deficiencies. In particular, relatively large proportions are necessary to produce adequate wetting of the finely divided inorganics. When used in large proportions, the wetting agents markedly detract from the properties of the finished composite. Coupling agents have been developed to overcome this difficulty. These fall into two main classes. The first, the more widely used, are trialkoxy organo functional silanes. Their activity is based upon chemical interaction between the alkoxy portion of the silane and filler and the chemical reaction of the organo functional portion with the polymer matrix. This provides a direct chemical link between the polymer and filler. But silanes have drawbacks. They are typically highly flammable, difficult to handle, and not easily worked into many polymer systems. Where the polymers do not contain functional groups or where the filler does not contain acidic protons, the silanes are often ineffective because of their inability to interact. For example, silanes are ineffective in thermoplastic hydrocarbons and fillers, such as carbon black, and to a large degree, calcium carbonate and sulfur.
The second group of coupling agents includes the organo-titanates which may be prepared by reacting tetraalkyl titanates with aliphatic or aromatic carboxylic acids. Of particular interest are the di- or trialkoxy acyl titanates or certain alkoxy triacyl titanates. These titanates, however, have serious drawbacks: e.g., they tend to decompose at temperatures frequently used in preparing many polymers; they tend to discolor certain inorganic materials used with polymer systems; and they are not compatible with many polymer systems.